In numerous industrial applications, it is desirable to sufficiently remove different contaminants from various metal, polymeric, ceramic, composite, glass, and natural material substrates. It is often required that the level of contaminant removal be sufficient such that the substrate can be subsequently used in an acceptable manner. Industrial contaminants which are typically removed include organic compounds (e.g., oil, grease, and polymers), inorganic compounds, and ionic compounds (e.g., salts).
In the past, halogenated solvents have been used to remove contaminants from various substrates and, in particular, chlorofluorocarbons have been employed. The use of such solvents, however, has been disfavored due to the associated environmental risks. Moreover, employing less volatile solvents (e.g., aqueous solvents) as a replacement to the halogenated solvents may be disadvantageous, since extensive post-cleaning drying of the cleaned substrate is often required.
As an alternative, carbon dioxide has been proposed to carry out contaminant removal, since the carbon dioxide poses reduced environmental risks. U.S. Pat. No. 5,316,591 proposes using liquefied carbon dioxide to remove contaminants such as oil and grease from various substrate surfaces. Moreover, the use of carbon dioxide in conjunction with a co-solvent has also been reported in attempt to remove materials which possess limited solubility in carbon dioxide. For example, U.S. Pat. Nos. 5,306,350 and 5,377,705 propose employing supercritical carbon dioxide with various organic co-solvents to remove primarily organic contaminants.
In spite of the increased ability to remove contaminants which have limited solubility in carbon dioxide, there remains a need for carbon dioxide to remove a wide range of organic and inorganic materials such as high molecular weight non-polar and polar compounds, along with ionic compounds. Moreover, it would be desirable to remove these materials using more environmentally-acceptable additives in conjunction with carbon dioxide.
In view of the foregoing, it is an object of the present invention to provide a process for separating a wide range of contaminants from a substrate which does not require organic solvents.